This invention relates to a controllable throttle assembly for a vacuum pump, particularly a cryopump.
The throttle assembly has a frame which is secured in a separate, generally circular flange and which supports strip-like elements (slats) rotatable about their longitudinal axes and are thus adjustable in their inclination. The inclined position of the slats may be adjusted by an actuating rod which passes through a radially oriented bore provided in the flange.
A throttle valve of the above-outlined type is disclosed, for example, in published European Application No. 102,787. The construction disclosed therein comprises sector-shaped slats which are arranged in a star-like manner and which are disposed within the flange for rotation about their longitudinal axis. The common actuation of the slats is effected by means of a cable which is guided over discs connected at their radially outer portion with the sector-shaped shutoff slats. One of the discs is provided with a shaft projecting outwardly through a bore in the flange. The throttle valve is actuated by rotating the shaft.
It is a disadvantage of a throttle valve of the above-outlined type that it is of relatively tall construction and requires a plurality of sector slats which are moved in a coordinated manner relative to one another. While it is feasible to reduce the number of the sector slats, such a solution would result in an even taller throttle construction because the smaller the number of the sector slats the greater the maximum width thereof. A sealing of the shaft which is required for the transfer of the rotary motion and which projects outwardly through the flange is wrought with difficulties in pumps where the usual high vacuum conditions must prevail. It is a further disadvantage of the above-discussed prior art construction that the cable which is exposed to significant temperature fluctuations does not operate reliably over an extended period of time.
In German Offenlegungsschriften (Non-examined Published Patent Applications) Nos. 2,936,931 and 3,216,591 throttles are disclosed which are fixedly installed in cryopumps having at least two stages. These throttles are in each instance situated in the zone of the inlet opening of the cryopump and have essentially two functions. First, the components of the throttle constitute pumping surfaces for gases with relatively high condensation temperatures (referred to hereafter generally as vapors), such as, for example, water vapor. In order to bind such vapors with cryo-condensation, the throttle is connected by means of a good heat conductor with the first stage of a refrigerator. As a result, the throttle has an operating temperature of approximately 50 K. to 80 K. Second, the fixedly installed throttles have the task to protect the inner pumping surfaces in case of the presence of excessive vapor. The inner pumping surfaces have a temperature of approximately 10 K. to 30 K. and serve essentially for the cryo-condensation of low boiling-point heavy gases (hereafter referred to as permanent gases), for example, argon. Additionally, on inner pumping surfaces an adsorption material is provided for adsorbing light gases such as hydrogen and helium. When a periodically significantly increased vapor proportion is present, for example, during sputtering processes (for which cryopumps are increasingly used), and if the vapors gain access to the inner pumping surfaces, the latter will be covered with ice in a short period of time and, as a disadvantageous result, the permanent and light gases can no longer be effectively pumped.
The throttle valve disclosed in German Offenlegungsschrift No. 2,936,931 is a shutter construction having a stator plate and a rotor plate. Each plate has spoke-like arranged sector-shaped openings whose magnitude is so selected that the openings of the stator plate may be opened or closed by turning the rotor plate. In case of an increased vapor presence the throttle is brought into its closed position whereby the inner pumping surfaces are protected. A throttle structured in this manner may, in its maximum open position, free at the most 40% of the inlet opening area of a cryopump thus presenting, even in the fully open state, a relatively large obstacle for the inflowing permanent gases. Further, a throttle of this type is not adapted to additionally perform the function of a baffle which has the task to protect the cold surfaces in the pump housing from direct heat radiation. If such a protection is desired, then, in addition to the throttle disclosed in German Offenlegungsschrift No. 2,936,931, a baffle must be used which provides for an "optical screening". The conventional baffles occupy approximately 50% of the inlet opening so that the baffle and the throttle together block 90% of the path leading to the inner cold surfaces. A further reduction of the evacuation capacity occurs by virtue of the necessity to arrange the baffle and throttle behind one another which results in relatively long pumping paths. The practically achievable permanent suction capacity of a cryopump equipped with a baffle and throttle is therefore, in the fully open state of the throttle, maximum 20% of the theoretically possible suction capacity.
A throttle which is fixedly installed in a cryopump, as disclosed in German Offenlegungsschrift No. 3,216,591 (FIGS. 3 and 4), includes parallel-arranged slats whose inclination may be varied. The actuation of the slats is effected by means of a cable which is guided into a lateral chamber through a nipple provided for this purpose at the pump housing. Within the chamber there is disposed a rotatable disc to which the cable is attached. In this prior art construction too, the cable exposed to the significant temperature fluctuations cannot reliably operate for an extended period. The actuating rotary component requires a vacuumtight rotary passage therefor. This construction further has the disadvantage that a cryopump having a fixedly installed throttle of the prior art structure is, as compared to a cryopump in which a controllable throttling of the suction capacity is not required, inordinately complex and thus expensive because, in particular, there are required a separate coupling nipple for guiding the cable, a vacuumtight rotary joint and an increased structural height.